By using Nintex Form templates you can get a consistent look across all forms being created.   See here for instructions:  

Hope that helps!

Mike

It only works for new forms. How do I apply it to existing forms? So back to my original question - global css?

Hi Igor, you can reference a custom CSS file and then make changes to that file only.  Go to form settings | advanced and one of the inputs is for custom css includes.

However for any existing forms, you will have to go and modify them individually to reference the file.  Once that is done you now have global control!

I suppose that SharePoint's "Alternate CSS" setting (under Site settings / Look and Feel / Master page) could host your site-scoped globals for Nintex as well as other SharePoint branding tweaks.

Also, there's always an option to reference custom CSS files by means of site or site collection scoped UserCustomActions targeting a ScriptLink or injecting a ScriptBlock, but that would require a client-side developer. More info and samples can be found at dev.office.com.

If you're looking to package it up outside of Nintex's form reference, the AlternateCSSUrl has been added to CSOM. Check out the below video and GitHub sample for more info:

PnP/Samples/Branding.AlternateCSSAndSiteLogo at dev · OfficeDev/PnP · GitHub 

if your forms are in one site collection, you can use AlternateCSS approach, if they are spread across multiple site collections you can try delegate controls, using control id "AdditionalPageHead" you can add css or js files through your control.